1. Field of the Invention
This invention relates generally exhaust systems. More particularly, the invention relates to an exhaust system for a multi-cylinder internal combustion engine.
2. Background
Motorcycles commonly employ exhaust systems to convey the exhaust gas from the engine's cylinder to the ambient environment. The journey begins at the engine cylinder, which incorporates intake and exhaust ports for ingress and egress to the cylinder. Fresh air mixed with fuel enters the engine cylinder through the intake port where it is subsequently compressed by a piston and ignited. A rapid expansion of the compressed fuel and air occurs, thereby forcefully moving the piston in the opposite direction to the compression stroke. Once the expansion is complete, the exhaust port opens to allow the combustion by-products or gas to exit the engine cylinder and enter an exhaust pipe. The exhaust port may be a passageway into the engine cylinder that is uncovered by the retreating piston, as in a two-stroke design well known in the art. In the case of a four-stroke design, a valve is utilized to open or close the exhaust port. The exhaust gas expelled from the engine cylinder, after passing through the exhaust port, enters an exhaust pipe. The exhaust pipe is designed to direct the exhaust gas towards the rear of the motorcycle and commonly utilizes bends and curves to accomplish this goal.
In a single cylinder engine, the exhaust gas, after passing through the exhaust pipe, is typically fed into a muffler prior to its expulsion into the atmosphere to dissipate unwanted noise originating in the combustion process. The exhaust system may also include a catalytic converter or other exhaust treatment device well known in the art. The muffler design will significantly affect the audible noise level or sound of the engine. A manufacturer can attenuate or change the sound of the engine so as to not only meet governmental noise requirements but also for the engine to exhibit a pleasing sound to the ear.
Depending on the design of the exhaust system, including the muffler and exhaust pipe, back pressure will be introduced, which impedes the free flow of exhaust gases along the exhaust system's entire length. For example, in a four-stroke engine, the piston pushes the exhaust gases out of the cylinder and into the exhaust system. If the back pressure in the exhaust system is reduced, the piston requires less force to expel the exhaust gases from the engine cylinder and thus increases the performance and efficiency of the engine. The performance of an engine is measured by the engine's generation of, for example, horsepower and torque over the entire rpm operating range. Generally, less back pressure will enhance the performance of the engine, or more specifically the engine's production of horsepower and torque while increasing its efficiency or reducing the engine's fuel consumption. However, a significant reduction in back pressure, which may be accomplished by, for example, using short exhaust pipes and no muffler, may have an adverse effect on engine noise and overall performance. An exhaust system design that maximizes the horsepower of an engine will often have a deleterious effect on the engine's torque production over a portion of the RPM range. If this drop in torque is located in the middle of the RPM range, it may be noticeable as a momentary drop in acceleration to the rider or driver and be undesirable.
The overall length and shape of the exhaust system is an important factor in determining how the engine will operate and affect the performance of the engine. For example, with a multi-cylinder engine it is commonly preferred to have individual exhaust systems for each cylinder so as to prevent any flow turbulence caused by pulses of exhaust gas from different cylinders combining before being expelled to the atmosphere. However, individual exhaust systems may not be feasible when the engine has more than two cylinders due to cost, size, weight, and packaging limitations. This concern is especially acute for a motorcycle since the exhaust system needs to fit close to the motorcycle frame so that the rider and passenger can straddle the motorcycle and not be subjected to burns or the like caused by contact with the hot exhaust system. An automobile is less prone to the concern for unwanted contact with the exhaust system as the car's floorpan is a barrier between the exhaust system and the occupants. A motorcycle, in a similar fashion, can incorporate bodywork to enclose the exhaust system to further protect the rider/passenger from the hot exhaust system. This bodywork may also act as a sound barrier to reduce the noise associated with the exhaust system. For an automobile, the length of the exhaust system may be increased to help dampen out the engine noise originating in the combustion process, but this may not be well suited for a motorcycle due to a motorcycle's relatively short length as compared to an automobile.
The geometry or cross-sectional area of the exhaust system may also be varied along its length to vary the engine's performance. An ever-increasing cross-sectional area in the exhaust system will decrease the chance of causing a significant increase in back pressure. A constriction at any point in the exhaust system will have an impact on the velocity of the pulses of exhaust gas throughout the entire exhaust system. But as with the length and shape of the exhaust system, continually increasing the cross-section in the exhaust system is difficult to accomplish due to, for example, packaging and manufacturing concerns. A compromise is to incorporate step increases in the cross-section of the exhaust system along its entire length. For example, a step increase in the exhaust system's inside diameter may be incorporated at each exhaust system flange connection from the exhaust pipe to the muffler.
Exhaust systems are commonly routed along the sides or below the motorcycle depending on such design factors as, for example, the orientation of the engine cylinders with respect to one another, the orientation of the engine in the motorcycle frame, the preferred riding characteristics, the size of the motorcycle, and the location of the motorcycle's center of gravity. For example, a motorcycle with an engine inline with the frame can easily route its exhaust system along the sides of the motorcycle due to its narrow width. When the engine is transverse to the frame, there will be less available space to route the exhaust system along the sides of the frame due to the engine's increased width. In this case, the exhaust system may be routed below the engine and frame without increasing the overall width of the motorcycle. As a result of the many tradeoffs associated with the design of an exhaust system, a manufacturer will choose an exhaust system that presents a compromise between these characteristics for the consumer. As discussed above, these characteristics may include, for example, cost, size, weight, engine noise, performance, and packaging limitations.
Customization of exhaust components by motorcycle riders, such as exhaust pipes and mufflers, is common in the aftermarket. Customization allows the owner to re-optimize the characteristics of their vehicle so as to maximize their own satisfaction. A successful customization leads to not only personal satisfaction of accomplishment, but also a feeling of attachment to the vehicle. Often, the replacement of a component made by the original equipment manufacturer (OEM) with an aftermarket part does not live up to expectations and will not be easily reversible once it is completed. This can lead to the owner incurring additional costs to reverse the modification. For example, the addition of a force air induction system to an automobile often requires the cutting of a hole in the hood over an engine. If the owner decided the additional noise outweighed the performance increase, the purchase of a new hood would have to be absorbed to reverse the modification. In the case of exhaust systems, incorporation of aftermarket components often requires cutting and welding of the OEM exhaust system. Exhaust pipes or other parts of the exhaust system are often cut with subsequent welding being performed to incorporate the aftermarket component. Thus, the level of financial risk being taken by the owner and difficulty in reversing the modification are increased.
In the case of the liquid-cooled, horizontally opposed six-cylinder Honda Gold Wing.RTM. engine, the design of the stock OEM exhaust manifold affects the motorcycle's performance by causing a perceptible drop in torque near the middle of the engine's RPM range at top-gear speeds of between 50 and 75 mph. This drop in torque translates into less top-gear roll-on power which impacts the rider's ability to pass traffic at highway speeds with nothing more than the flick of the wrist. Except for this perceptible drop in torque, the Gold Wing's.RTM. smoothness and expansive torque make it one of the most rideable and satisfying machines on the market. Any potential aftermarket fix for this performance issue is further complicated by the Gold Wing's.RTM. OEM hidden exhaust system which is enclosed by an OEM shroud, thus impeding any modifications to the hidden exhaust system without permanent removal of the OEM shroud.